Exhaust gas purification catalyst

ABSTRACT

The present invention provides an exhaust gas purification catalyst which has a structure which prevents competitive adsorption of HC, CO, and NO x  and enables effective utilization of an NO x  storage reduction type catalyst. The exhaust gas purification catalyst of the present invention is characterized by having an NO x  storage reduction type catalyst layer which contains at least one type of NO x  storage material which is selected from an alkali metal or an alkali earth metal and Pt and/or Rh on a substrate and having an oxidation catalyst layer which carries Pt and/or Pd on the NO x  storage reduction type catalyst layer.

TECHNICAL FIELD

The present invention relates to an exhaust gas purification catalyst,more particularly relates to an exhaust gas purification catalyst whichis provided with an NO_(x) storage reduction type catalyst layer.

BACKGROUND ART

In the past, in three-way catalysts, NO_(x) storage reduction typecatalysts have suffered from competitive adsorption of HC, CO, andNO_(x), so it has been difficult to secure sufficient purificationperformance.

To solve this, Japanese Patent Publication No. 2009-101252 A1 etc.report NO_(x) storage reduction type catalysts which have two-layercoated structures, but both the upper and lower layers contain an NO_(x)storage material (or NO_(x) holding substance), so the problems thatcompetitive adsorption of HC, CO, and NO_(x) occurs, the active pointsof the NO_(x) storage reduction reaction end up decreasing, and anNO_(x) storage reduction type catalyst cannot be effectively formed wentunresolved.

SUMMARY OF INVENTION

The present invention has as its object the provision of an exhaust gaspurification catalyst which has a structure which prevents competitiveadsorption of HC, CO, and NO_(x) and enables effective utilization of anNO_(x) storage reduction type catalyst.

To achieve the above object, the exhaust gas purification catalyst ofthe present invention is characterized by:

having an NO_(x) storage reduction type catalyst layer which contains atleast one type of NO_(x) storage material which is selected from analkali metal or an alkali earth metal and Pt and/or Rh on a substrateand

having an oxidation catalyst layer which carries Pt and/or Pd on theNO_(x) storage reduction type catalyst layer.

In a preferred embodiment, seen in the direction of flow of exhaust gas,the oxidation catalyst layer has a length of 25 to 60% of the length ofthe NO_(x) storage reduction type catalyst layer.

In a preferred embodiment, the oxidation catalyst layer has a thicknessof 20 to 40 μm.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 schematically show HC, CO, and NO_(x) which suffer fromcompetitive adsorption at an NO_(x) storage reduction type catalystlayer in a conventional exhaust gas purification catalyst.

FIG. 2 schematically shows an exhaust gas purification catalyst of thepresent invention in a state where HC and CO are selectively oxidized atthe upper layer oxidation catalyst layer and NO_(x) is selectivelystored and reduced at the lower layer NO_(x) storage reduction typecatalyst layer.

FIG. 3A is a schematic view which shows an image of composition ofcatalysts of invention examples and comparative examples for laboratoryevaluation use and FIG. 3B is a schematic view which shows an image ofcomposition of catalysts of invention examples for actual evaluationuse.

FIG. 4 is a graph which shows a test cycle for laboratory evaluation.

FIG. 5 is a graph which shows an NO_(x) storage characteristic in thelaboratory for catalysts of invention examples and comparative examples.

FIG. 6 is a graph which shows NO_(x) storage characteristics in anactual machine for catalysts of invention examples.

DESCRIPTION OF EMBODIMENTS

In a conventional two-layer coat structure, both the upper and lowerlayers are NO_(x) storage reduction type catalyst layers, so, as shownin FIG. 1, competitive adsorption of HC, CO, and NO_(x) ends upoccurring, the active points of NO_(x) storage reduction reactions arereduced, and the NO_(x) storage reduction type catalyst layer can beeffectively utilized. That is, even if the NO_(x) storage reduction typecatalyst has a sufficient NO_(x) storage capacity, the NO_(x) storagespeed at the stage of the start of storage is slow and, with modeemission, sufficient performance could not be exhibited. Further, as ameasure, even if arranging an oxidation catalyst upstream of the NO_(x)storage reduction type catalyst, when compared with the same capacity,again the NO_(x) storage speed was slow.

As opposed to this, as a characterizing feature of the presentinvention, as shown in FIG. 2, first, HC and CO are substantiallyremoved at the upper layer oxidation catalyst layer, so the activepoints of the lower layer NO_(x) storage reduction type catalyst layercan be effectively utilized for the NO_(x) storage reduction reaction.

The present invention can provide an oxidation catalyst layer on anNO_(x) storage reduction type catalyst layer so as to enable effectiveutilization of the lower layer NO_(x) storage reduction type catalystlayer. That is, the upper layer oxidation catalyst layer through whichthe exhaust gas first passes selectively oxidizes the HC (hydrocarbons)and CO (carbon monoxide) which inhibit the reaction of the NO_(x)storage reduction type catalyst. Therefore, in the lower layer NO_(x)storage reduction type catalyst layer, competitive adsorption of HC, CO,and NO_(x) substantially does not occur. Storage and reduction of NO_(x)which passes through the upper layer oxidation catalyst layerselectively occurs.

In the exhaust gas purification catalyst of the present invention, byselectively causing action of the upper layer oxidation catalyst layerand the lower layer NO_(x) storage reduction type catalyst layer, it ispossible to avoid competitive adsorption of the HC and CO which shouldbe removed by oxidation and NO_(x) which should be removed by reduction,secure sufficient reaction sites, and enable maximum utilization of theinherent functions of the NO_(x) storage reduction type catalyst.

In one preferable embodiment, as shown in FIG. 2, the oxidation catalystlayer is arranged along the direction of flow of the exhaust gas fromthe upstream side end of the NO_(x) storage reduction type catalystlayer so as to cover 25 to 60% of the length of the NO_(x) storagereduction type catalyst layer. In this range, the NO_(x) storage speedof the NO_(x) storage reduction type catalyst layer becomes thegreatest.

Further, in one preferable embodiment, the thickness of the oxidationcatalyst layer is 20 to 40 μm. In this range, the NO_(x) storage rate ofthe NO_(x) storage reduction type catalyst layer becomes maximum.

After the lower layer NO_(x) storage reduction type catalyst layer isformed, the upper layer oxidation catalyst layer is overcoated on it toform a catalyst of a vertical two-layer coat structure.

The oxidation catalyst layer which is overcoated on the upper layer israised in oxidation performance by being provided with Pt and/or Pdwhich has excellent catalyst ability as an oxidation catalyst.

The oxidation catalyst layer does not have Rh inhibiting the catalystactivity in lean combustion gas added to it, while does not have anNO_(x) storage material which affects the previous metal activity addedto it either.

The NO_(x) storage reduction type catalyst layer includes Pt and/or Rhand an NO_(x) storage material. In particular, Rh and the NO_(x) storagematerial are also added to only the lower layer NO_(x) storage reductiontype catalyst layer.

EXAMPLES

Three types of exhaust gas purification catalysts which have the coatspecifications which are shown in Table 1 on cordierite substrates wereprepared. Below, DOC indicates an oxidation catalyst layer, while NSRindicates a NO_(x) storage reduction type catalyst layer.

TABLE 1 Name Coat specifications [1] DOC Coat: Al₂O₃ = 150 *Unit:g/liter (Comp. ex.) Precious metal: Pt/Pd = 1.2/0.6 *Unit: g/literOxidation catalyst NO_(x) storage reduction layer (upper layer) typecatalyst layer (lower layer) [2] NSR None Coat: CeO₂•Al₂O₃ = 120, (Comp.ex.) ZrO₂•TiO₂ = 100 ZrO₂•CaO = 50 *Unit: g/liter Precious metal: Pt/Rh= 2.0/0.45 *Unit: g/liter Storage material: Ba/Li/K = 0.1/0.2/0.1 *Unit:mol/liter [3] Coat: Al₂O₃ Same as above Overcoat *Coat amount differsNSR by length (Inv. ex.) Precious metal: Pt/Pd = 1.2/0.6 *Unit: g/literCoat ratio: 27, 55, 82% Coat thickness: 30 ± 10 μm

Each invention example was prepared by forming an NO_(x) storagereduction type catalyst layer (NSR) on a substrate, then coating anoxidation catalyst layer (DOC) on the same.

The catalyst size was, for laboratory use, a volume of 35 cc (totallength 50 mm) and for actual use, a volume of 14 liters (total length110 mm).

Each catalyst was tested for simple durability in an electric furnace at700° C.×27 hours.

Three types of catalysts of the coat specifications of Table 1 were usedfor tests under the following conditions. The images of composition areshown in FIG. 3.

<Configuration of Test Catalysts>

(A) Configurations for Laboratory Evaluation Use (Three Types) [1]DOC+[2]NSR: Tandem Configuration (Comparative Example)

-   -   Size: DOC volume 10 cc, length 14 mm (upstream side)        -   NSR volume 25 cc, length 36 mm (downstream side)            -   (Total length 50 mm)

[2] NSR: Alone (Comparative Example)

-   -   Size: Volume 35 cc, length (total length) 50 mm

[3] Overcoat of DOC on NSR (Invention Example)

-   -   Size: Volume 35 cc, length (total length) 50 mm    -   Overcoat ratio: 27%=10 cc (14 mm(*))        -   55%=20 cc (28 mm(*))        -   82%=30 cc (42 mm(*))        -   (*) Length from upstream side end of NSR

(B) Configuration for Actual Evaluation Use (One Type) [3] Overcoat ofDOC on NSR (Invention Example)

-   -   Size: volume 14 liter, length 110 mm    -   Overcoat ratio: 55%=7.7 liter (60 mm (*))        -   (*) Length from upstream side end of NSR

The test conditions were as follows:

<Test Conditions>

(A) Laboratory NO_(x) Storage Test

Test gas conditions: Shown in Table 2.

TABLE 2 Total flow CO₂ O₂ NO C₃H₆ H₂O rate Lean 10% 10% 100 ppm  300 10%20 liter/min atmosphere ppmC (N₂ balance) Rich 10% 1% 100 ppm 10000 10%20 liter/min atmosphere ppmC (N₂ balance)

Test cycle: Shown in FIG. 4.

That is, the catalyst was raised from the initial temperature of 50° C.to 600° C. by 40° C./min. At 600° C., rich spike NO_(x) reduction(rich/lean=5 sec/5 sec) was performed, then the catalyst was immediatelycooled in an argon atmosphere down to 350° C. where NO_(x) storage wasperformed in a lean atmosphere.

(B) Actual NO_(x) Storage Test

Evaluation engine: Diesel engine (exhaust amount: 2.2 liters)

Engine conditions: Shown in Table 3.

TABLE 3 Catalyst entry Inflowing Inflowing Speed temperature Ga NO_(x)THC 2000 rpm 370° C. 35 g/sec 100 ppm 135 ppmC

Evaluation pattern: PM regeneration→saturated NO_(x) storage amountmeasurement

<Test Results>

(A) Verification of Overcoat Ratio

FIG. 5 shows all together the results of the laboratory NO_(x) storagetest.

According to the present invention, by overcoating DOC on the NSR, theNO_(x) storage speed is remarkably improved compared with theconventional NSR alone and DOC/NSR in tandem.

Further, the NO_(x) storage speed becomes the highest in the range of 25to 60% of the NSR length of the lower layer of the overcoat ratio.

(B) Verification of Coat Thickness

FIG. 6 shows all together the results of the actual NO_(x) storage test.The overcoat ratio was 55% (fixed), while the overcoat amount waschanged in the range of 24 to 72 g/liter.

When the overcoat amount was near 30 g/liter, the NO_(x) storage speedwas the highest. The optimum overcoat amount is in the range of 25 to 35g/liter centered about 30 g/liter. If converting this to the overcoatthickness with respect to an overcoat ratio of 55%, the optimum overcoatthickness is 20 to 40 μm or so.

INDUSTRIAL APPLICABILITY

According to the present invention, there is provided an exhaust gaspurification catalyst which has a structure which prevents competitiveadsorption of HC, CO, and NO_(x) and enables effective utilization of anNO_(x) storage reduction type catalyst.

1. An exhaust gas purification catalyst characterized by having anNO_(x) storage reduction type catalyst layer which contains at least onetype of NO_(x) storage material which is selected from an alkali metalor an alkali earth metal and Pt and/or Rh on a substrate and having anoxidation catalyst layer which carries Pt and/or Pd on said NO_(x)storage reduction type catalyst layer.
 2. An exhaust gas purificationcatalyst as set forth in claim 1, wherein said oxidation catalyst layercovers 25 to 60% of the length of said NO_(x) storage reduction typecatalyst layer from an upstream side end of said NO_(x) storagereduction type catalyst layer along the direction of flow of the exhaustgas.
 3. An exhaust gas purification catalyst as set forth in claim 1 or2, wherein said oxidation catalyst layer has a thickness of 20 to 40 μm.